The present invention relates generally to improvements in television receivers, and more particularly to vertical phase-lock systems for locking the receiver's vertical scan to the frequency and phase of received vertical sync (synchronization) pulses.
U.S. television standards call for a nominal line rate of 15,750 H.sub.z and a frame rate of 30 H.sub.z. Hence, standard U.S. television signals may be said to have a nominal line rate/frame rate ratio of 525. This known ratio is employed in conventional synchronization systems for controlling the vertical scan of a television receiver. Typical of such conventional systems is one which generates clock pulses (2H pulses) of twice the standard television line frequency and counts 525 of such pulses to locally generate a vertical sync pulse. To lock the phase of the locally generated sync pulse to the phase of a sync pulse received as part of the composite television signal, a phase detector looks for coincidence between the locally generated sync pulse and the received sync pulse. When coincidence is detected, the locally generated sync pulse is employed to restart the counting of the 2H pulses and to initiate vertical scan. When a lack of coincidence is detected, the received sync pulse is utilized to restart counting of the 2H pulses and to initiate vertical scan. Typical of such systems are those disclosed in U.S. Pat. Nos. 3,691,297 and 3,916,102.
One situation which troubles synchronization systems of the type described above occurs when a non-standard television signal is received. For example, a non-standard signal may include vertical sync pulses which are related to twice their line frequency by a factor of 525.5 rather than the nominal factor of 525. When such a non-standard signal is received, the conventional system may oscillate between utilization of its locally generated vertical sync pulses and the received vertical sync pulses, thereby permitting the image generated by the television receiver to roll.
To offset the effect described above, it has been proposed to include a signal detector which examines the received television signal for the presence of equalizing pulses adjacent the received vertical sync pulse. When equalizing pulses are detected, the assumption is made that a standard television signal is being received and the system is permitted to operate normally as described above. When no equalizing pulses are detected, the assumption is made that a non-standard signal is received and the system is constrained to employ the received sync pulse for initiating vertical scan. Hence, the oscillation described above is avoided.
However, two drawbacks accompany the use of the signal detector described above. First, such a detector is complex and costly. In addition, the assumption that the presence of equalizing pulses indicates a standard television signal is not always correct. For example, some non-standard sources such as various tape record/playback machines provide equalizing pulses in their composite television signal. Consequently, the conventional system is "fooled" by the presence of such equalizing pulses, with the result that it alternates between using its locally generated sync pulses and the received sync pulses for initiating vertical scan. Because the locally generated sync pulse is not in exact time coincidence with the non-standard received sync pulse, the television picture rolls vertically. Hence, prior digital vertical synchronization systems have not been only relatively costly, but they have not consistently and reliably provided proper vertical scan in television receivers receiving non-standard signals.